1. Field of the Invention
This invention relates to static vent units and particularly to such units for use on helicopters.
2. Description of the Prior Art
Static vent units are used on both fixed and rotary wing aircraft to sense the ambient pressure of the air through which the aircraft is flying. This provides an indication of the altitude at which the aircraft is flying and, in combination with a pressure signal sensed by a forward facing pitot tube, an indication of the airspeed and, for fixed wing aircraft, the Mach number.
For this reason, static vent units have ideally to be located in a position where the local ambient pressure varies only with altitude and not with other features such as airspeed and relative wind direction, and are therefore usually located in the sides of an aircraft fuselage.
It has been proposed to locate such a sideways facing static vent unit in the nose of a helicopter in order to reduce the weight and complexity associated with long pipe runs to the more usual location in a helicopter tail boom. The nose location is subjected during certain phases of operation, e.g. the hover, to below ambient pressures, and a strake device has been located beneath the vent port to increase the pressure at the vent port and compensate for the negative pressure at the vent port location.
During a flight test of a helicopter fitted with such a static vent unit involving a high rate of descent, it was noted that the forward speed appeared to be slower than that indicated in the cockpit. Subsequent wind tunnel tests indicated that when subjected to positive airflow incidence (relative airflow from below) such as occurred during the descent, a pressure coefficient above the strake in the area of the vent port reduced significantly causing an incorrect indicated airspeed on the cockpit instruments.
U.S. Pat. No. 3,612,439 discloses a static vent unit for a high performance fixed wing aircraft designed to operate over a wide Mach number range extending to supersonic values. The unit includes compensation means in the form of an exterior shallow bulge having a thickness which decreases smoothly from a central region to the edges and includes a static vent formed by an aperture located in the surface of the bulge itself and offset in respect of a major axis of the vent unit.
The prior device operates to modify the airflow locally in operation so as to compensate a position error arising from variation in Mach number, and in respect of airflow from only one of two generally perpendicularly opposed directions.
The aforementioned problem associated with the inaccuracy of the sensed pressure from a static vent unit on a helicopter stems from the wide range of operating conditions of which the helicopter is capable. This means that the direction of relative airflow changes significantly resulting in a very wide airflow incidence range in which the static vent unit is required to operate.
An objective of this invention therefore is to provide in or for a helicopter a static vent unit which overcomes the aforesaid problem.